Sunday 27 May 2012 11.56 EDT
First published on Sunday 27 May 2012 11.56 EDT

New evidence has emerged claiming that members of Bashar al-Assad's family and inner circle are directly ordering the commission of crimes against humanity in Syria. Experts in international law consider it "preposterous and completely implausible" that the president would be unaware of systematic and widespread killing and torture.

Defectors from Syrian intelligence and security agencies, used by the regime to crush the revolt, claim that Assad's cousin issued shoot-to-kill orders against civilian protesters in Dera'a, the cradle of the insurrection. Kill quotas were reportedly issued to snipers tasked with assassinating pro-democracy activists.

They allege that Assad's brother Maher, a senior army commander, was among senior figures operating out of a secret command centre in Dera'a when orders were issued to contain a protest march by all means necessary. More than 100 civilians were shot dead. In addition, Maher is accused of ordering the indiscriminate mass punishment of the entire male population of a troublesome town, al-Moudamya, last year.

The defectors' testimony, to be broadcast by Channel 4 Dispatches on Monday night, has added resonance after a weekend when arguably the ugliest atrocity of the 15-month confrontation was perpetrated at Houla. Though the regime blamed rebels, western powers are adamant that regime tanks were responsible.

In the documentary, The Real Mr & Mrs Assad, footage previously unseen in the UK shows the president saying: "Every mistake [that] happens in this government, you are responsible, not somebody else. Not the minister. Not the prime minister. At the end you should be responsible." In more recent months, Assad has repeatedly denied any role in the killings.

"No one is authorised to give orders to the security forces except for him," said the exiled former Syrian vice-president Abdul Halim Khaddam, who fell out with Assad in 2005, having served under him and his father. Speaking in Paris, where he lives, Khaddam said: "Will anyone believe that 300,000 soldiers can come out of their barracks to slaughter citizens due to an initiative by their officers? These orders are issued by the president of the state."

The witness accounts follow the publication by the Guardian in March of emails downloaded by activists from the private accounts of Assad and his London-born wife, Asma. The emails revealed that the president was kept personally informed about military deployments in the city of Homs, where an estimated 4,000 died during weeks of relentless bombardment.

The Dispatches documentary further examines emails from the same cache that indicate the Assads were aware of the arrest of individuals as part of the crackdown. In two separate cases, they appear to have personally intervened to secure the release of detainees.

One email, dating from mid-December last year, was sent to both Bashar and Asma al-Assad by her father, the London-based cardiologist Fawaz Akhras. It drew their attention to a Channel 4 documentary, Syria's Torture Machine, which was about to be aired. He suggested that the Syrian government respond to the allegations that state-sanctioned torture was systematic and widespread.

The Syrian embassy in London did respond, before the programme was broadcast, using an almost identical form of words to those suggested by Akhras in his email.The defectors also detail allegations against Brigadier-General Atef Najib, Assad's cousin, who headed the political security directorate in Dera'a at the start of the uprising. Two defectors claim to have received direct orders from Najib to fire live ammunition at demonstrators.

One of them, Amar Sheikh, formerly of central security in Damascus – who was drafted in to Dera'a – described Najib telling a briefing which he attended: "You who are armed, you are required if you see that the situation is getting out of control to shoot the demonstrators with bullets."

The bloody crackdown in Dera'a caused national outrage and was raised in parliament by a clearly angry Dera'a MP – after which Najib was transferred.

"Clearly this decision was made very high up," said Anna Neistat of Human Rights Watch, an expert in the command structure of the Syrian armed forces. "He is the president's cousin. I think there was clearly awareness that something he did was bad enough that it could lead to a major revolt."

Another defector, Afaq Ahmed, a former member of the special operations directorate of the feared air force intelligence branch, claims that a later killing spree, in which 100 protesters were shot dead, was approved by the head of air force intelligence. He was allegedly based in a secret command centre in the Kuwait hotel on the city's eastern outskirts.

It is understood Maher al-Assad was a member of his brother's inner circle decamped to Dera'a and had based in this command centre. The European Union describes Maher as "the principal overseer of violence against demonstrators." Ahmed, who is in a secure location in a neighbouring country, said: "Our task was restricted to assassinating activists and protesters based on orders and the permitted killing quota authorised by the authorities. The quota varied. Some days it was 10; others, 15 or 20."

This accords with evidence gathered by Neistat. "There does appear to be a policy on how to crack down on the protests," she said. "We interviewed one of the snipers … and he said that before the protest they were given a specific percentage – essentially a quota – on how many people, in relation to the overall number of protesters in the streets, they were allowed to take down."As yet, there is no smoking gun linking Assad directly to the commission of crimes. But William Schabas, a professor of international criminal law, said that even without "a signature on direct orders" he believes there is now sufficient evidence to hold the president to account, using the doctrine of command responsibility."We can hold him responsible," Schabus says, "even if we can't prove that he actually ordered the crimes. Whether he is a micro-manager of atrocity, or whether he's a macro-manager, it doesn't actually make much difference.

"Hitler was a macro-manager. There is very little evidence of Hitler ordering direct atrocities to be perpetrated. Does anyone have any doubt that Hitler wasn't in charge? I don't think so. And I think that this is a similar case."

Last month, the former US Middle East peace envoy, George Mitchell, said Assad could be put on trial in the same way as Charles Taylor, the former president of Liberia, who was found to have "aided and abetted" war crimes by a UN-backed tribunal in The Hague.

Any future prosecution of Assad on charges of crimes against humanity must be unanimously referred by the United Nations security council to the International Criminal Court – as happened in the case of Libya last year. At present, this is considered unlikely, owing to the likelihood of a Russian and Chinese veto. But the British government, among others, is assisting in the gathering of evidence which could one day be used in court.

"We will do everything within our power," says the Foreign Office minister Alistair Burt. "We have seen former leaders of regimes in the ICC already, so no one can discount the possibility that it may well happen and if you were Bashar al-Assad, you would not bet that it would not happen to you."

Jonathan Miller is foreign affairs correspondent with Channel 4 News. His Dispatches, The Real Mr & Mrs Assad, will be broadcast on Channel 4 at 8pm on Monday night.